j. Raptor Res. 28(3):154-157 ¸ 1994 The Raptor Research Foundation, Inc. NEST-BOX VERSUS NATURAL-CAVITY NESTS OF THE EASTERN SCREECH-OWL: AN EXPLORATORY STUDY FREDERICK R. GEHLBACH Departmentof Biology,Baylor University, Waco, TX 76798 U.S.A. ABSTI•½T.--The use of nest boxesversus natural tree cavitiesby easternscreech-owls (Otus asio) in centralTexas was exploredfor 9 yr. Box sitesresembled natural-cavity sites vegetatively and physically exceptthat the boxeswere positionedsomewhat lower. The three box sizesprovided spanned the size rangeof natural cavitiesused by screech-owls.Box sizemade no differencein nestsite selection,clutch size, or nestingsuccess. Three kinds of wood usedin box constructionmade no differenceeither. The largestboxes were usedmore often for replacementnests, and the smallestones tended to crowdbroods, contributingslightly to mortality and early fledging.Overall, nestboxes produced data on frequencyof use, clutch size, and fledgling productivity that were equivalent to data from natural cavities. KEY WORDS: breedingsuccess; eastern screech-owl; nest boxes; nest-site use; Otus asio;tree-cavity nests. Nidos en cajasanideras versus cavidades naturales de Otusasio: un estudioexploratorio RESUMEN.--Seexplor6 por nueveaftos el uso de cajasanideras versus cavidades naturales ubicadas en firbolespor parte de Otusasio en el centro de Texas. Las cajasanideras se asemejaronalas cavidades naturalestanto vegetativacomo fisicamente, excepto que las cajasfueron ubicadasalgo mils abajo.E1 tamafiode las cajasutilizadas correspondla al espectrode tamafiosde las cavidadesnaturales usadas por O. asio;no se registrarondiferencias en la selecci6nde sitio de nidificaci6n,tamafio de la nidada o •xito de lospolluelos. Tres tiposde maderasse utilizaron en la construcci6nde las cajasy tampocose registraron diferencias.Las cajasmls grandesfueron utilizadasa menudopara reemplazode nidosyen las mils pequefiasse tendla a agrupar las crlas, aumentandoligeramente la mortalidad y un desarrolloprecoz. En general,las cajasanideras producen datos sobre la frecuenciade uso,tamafio de nidaday productividad de volantonesque fueron equivalentesa los obtenidosdesde cavidades naturales. [Traducci6n de Ivan Lazo] Many raptor researchersemploying artificial owls, only the boreal or Tengmalm'sowl (Aegolius structures like nest boxes or nesting platforms do funereus) has been studied in a somewhat similar not utilize or report simultaneouscomparisons with manner; clutchsize and number of fledglingsof that natural structuresor distinguishsite suitability or specieswere determinedin both nestboxes and nat- availability in studying habitat selection.Moller ural cavities but not in controlled fashion (Korpi- (1989, 1992) and Clobert and Lebreton (1991) note mSki 1984). possibledeficiencies in suchstudies using nest boxes, METHODS includingthe potentialfor greaternesting success in The study was conductedin 135 ha of Woodway, a boxes than natural cavities. Here, I focus on the suburb of Waco, TX, U.S.A., during 1967-75. This area simultaneous comparison of eastern screech-owls had 508 human residents/km2 and 25.9% green space (Otusasio) nesting in boxesand natural tree cavities averaging 327-1504 trees/ha in lawns and wooded ra- to discover if there was a difference in the use of and vines,respectively. Eastern screech-owls and the natural tree cavitiesthey usedwere locatedby mappingcavity- breedingsuccess in the two site types. advertisementsongs and making cavity inspectionsin Field studiesof birds may need to be conducted Decemberthrough early March before nestingbegan. I over severalyears during which investigativebiases continuedto searchfor used and unusedbut apparently canmultiply errors.Consequently, I testedfor biases suitable cavities (as large or larger than used cavities; McCallum and Gehlbach [1988]) until the cumulative that might result from using nest boxes prior to number found versus cumulative search effort indicated conductinga 16-yr populationstudy of the eastern that essentiallyall were known. screech-owl(Gehlbach 1994). Amongcavity-nesting Nine nest boxes were constructed,three each of exterior 154 SEPTEMBER 1994 SCREECH-OWL BOX AND CAVITY NESTS 155 plywood,solid pine, and solidcedar (1.9-cm-thick wood), RESULTS and were painted dark brown on the outside.Cavity size has beenshown to influenceclutch size in owls (Korpim5ki Nest Cavities and Habitat. Of the 23 suitable 1985), and to test its affect on screech-owlsI built one box of each wood type with 225, 400, and 625 cm2 bottoms. natural cavities that I found, eastern screech-owls All boxes had a 6.8 cm entrance hole 25 cm above the box nestedin 15 that were deeper(>25 cm) with larger bottom.The bottomareas of theseboxes spanned those of floors(> 10 cm minimum dimension)and had small- natural cavities.Depth of the boxeswas the averageof er entrances (<15 cm maximum dimension) than 12-58-cm-deep natural cavities,and the entrancediameter was the mean minimum dimension of the natural entranc- the others(MANOVA F = 2.9, P = 0.04). The used es. siteswere 3.7 m (SD = 1.6) above ground in nat- Box locationsdepended on landowner permissionand urally rotted, hollow limbs or tree trunks. At least wereplaced 70-300 m fromnatural cavities used by screech- 10 of thesecavities had beenenlarged by fox squir- owls and other boxeson straight tree trunks. Trunk di- rels. The red-bellied woodpecker(Melanerpes car- ameters at these positionswere equal to or larger than box width. Boxes faced nine different directions and were olinus),the largestlocal woodpecker species, did not 3-4 m above ground. Five boxeswere paired with the excavatecavities large enoughfor screech-owls. closestpreviously used natural cavitiesmost like them in Paired box- and natural-cavitysites were 72-280 orientation, height, and volume. The other four could not m apart and similar in their vegetationaland phys- be paired becauseit requiredseveral years to discoverall natural sitesused by screech-owls.I evaluated only first- ical features (MANOVA F = 1.1, NS). However, nest data, sincereplacement nests are so different (Gehl- the boxes tended to be lower (• -- 3.1 vs. 3.7 m, bach 1994). ANOVA F = 3.5, P = 0.06) and in smaller diameter A 75-m, 5-point,20-quarter transect, randomly aligned trees (• = 26.7 vs. 32.2 cm, F = 2.4, P = 0.09). througheach box and cavitytree, was employedto assess While all nine nestboxes were placedlower in small- vegetationalfeatures according to Gehlbach(1988, 1994). These and physicalmeasurements were made before the er trees than the 15 used natural cavities (ANOVA boxeswere positioned,so that box sites would resemble F = 3.2, P < 0.05), both site-typeswere equally natural cavity sites. Measurementsand synthesescon- distributedamong the sevenmost commontrees of cernedtree and shrubdensity, height, and diversity,the the canopy(X 2 = 1.7, NS). Cedar elm (Ulmus cras- evergreenfraction, canopy coverage, tree speciesrelative sifolia) was the dominant tree and had the most nat- importance, nest tree diameter, nest height and bottom area, and distanceto the nearesthouse, permanent water, ural cavities(48%) and boxes (44%). and suitable/availablecavity or box. The frequencieswith which screech-owlsused Severalhandfuls of dried leaveswere placedin the bot- paired boxesand natural cavitiesfor nestingwere tom of each box to simulate an old fox squirrel (Sciurus quite similar, and similar to all nine boxesand nine niger) nest becauseall but one of the usednatural cavities had them. Thereafter, box and cavity contentswere un- cavitieswith at least 5-yr records(Table 1). Type disturbedexcept in a minor way during nestinspections. of wood and box size made no difference in nest box Fox squirrelsand other cavity usersand nest predators selection;useage relative to availability rangedfrom were not disturbedduring the weeklysurveys of nestcon- 60% in pine and 67% in small boxesto 70% in cedar tentswhich extendedthrough June, and nest debriswas not removed between years. and 73% in large boxes(X 2 = 1.7, NS). Multifactor box versus natural-cavity environments, Preferred nesting habitat was distinguishedas high- versuslow-use boxes,and used versusunused nat- having more evergreensin the canopy, lower tree ural cavitieswere evaluatedwith multivariateanalyses density, closer alternate nest site, and lower shrub employingtransformed data. Individual parameterswere densityin first-to-lastorder of importance(F-- 4.0, then testedwith univariate analysisof variance.Use and reproductivedata for the box/cavitypairs were assessed P < 0.07). This combination of features describesa with Wilcoxon signed-rankstests, while unpaired data shady,park-like landscapewith large, cavity-prone were subjectedto Mann-Whitney U-tests.Spearman rank trees at low densities. These habitat characteristics correlationswere used to test relationshipsbetween box were identifiedby subjecting12 high- and 12 low- sizeand nestcontents. Chi-square analyses were madeof nests in boxes versus cavities, use of and results from the use nest sites(employing the 60% median use rate different box sizesand woodtypes, and box versuscavity of all nine boxesplus 15 cavities)to stepwisedis- dispersionsamong dominant trees. criminant analysis. Means,standard deviations, sample sizes (if notobvious Reproduction.Clutch size and numberof fledg- from the designs),and exactprobabilities are givenexcept lings/clutchin the nestboxes were statisticallylike for those>0.10, which I considernon-significant (NS) in two-tailedtests. Probabilities of <0.10 havepotential bi- those